The invention relates to a seat-occupancy and occupant-class detector for a motor-vehicle seat having a conductive surface element that mounted on insulation on a seat part of the vehicle seat roughly parallel to the seating surface thereof and that can be fed a measuring signal, and a sensor arrangement in a back part of the vehicle seat and that can detect the measuring signal applied to the conductive surface element on the seat part.
Seat-occupancy and occupant-class detectors of this kind are used in conjunction with smart airbag systems to ensure airbags are actuated as required. Seat-occupancy and occupant-class detectors of this kind are often used in the passenger seat.
Seat-occupancy and occupant-class detectors of this kind are intended to ensure by their passenger classification devices that when the passenger seat being occupied by a vehicle passenger, it is possible to determine what kind of person is sitting in the passenger seat, and it should moreover be possible to detect the sitting position of the person concerned in the passenger seat. The person located in the passenger seat should be classifiable in different passenger classes.
It is also possible in principle that with the seat-occupancy and occupant-class detector a vehicle passenger is reminded to fasten a seat belt, for which purpose the seat-occupancy and occupant-class detector can be connected to a seat belt reminder (SBR).
Classification of the vehicle passengers is undertaken in order to be able to control adaptive restraint system components depending on the passenger class determined for the person occupying the vehicle seat in each case.
The following passenger classes are employed in this case:                Class 0: Empty seat        Class 1: 1-year old child in child seat or younger        Class 1 or Class 2: Person between a 1-year old child and a 5th percentile female        Class 2: light adult≥5th percentile female        Class 3: heavy adult≥50th percentile male        
The 5th percentile female is a statistical measure for a woman below which 5% of all women fall. A 50th percentile male is a statistical measure for a man that is exceeded or not reached by 50% of men.
Depending on the passenger class determined for the vehicle passenger occupying the vehicle seat, the seat-occupancy and occupant-class detector according to the invention supplies a defined output signal. This output signal is fed to an airbag control device that performs the activation or deactivation of the passenger airbag.
Insofar as the vehicle passenger concerned falls into classes 0 or 1, the corresponding passenger airbag is deactivated. If the vehicle passenger concerned falls into classes 2 or 3, the passenger airbag is activated.
For seat occupancy detection and passenger classification de vices of this kind for vehicle seats, different sensory principles are applied. These involve camera systems, ultrasound systems, weighing sensors and the like, for example.
Capacitive measuring devices are frequently used on account of their simple design and reliability. An insulated conductive surface is integrated in a seat part of the vehicle seat. This creates capacitance relative to the vehicle mass. As soon as a vehicle passenger approaches this conductive surface, the capacitance thereof changes. The capacitance is transformed by charge sensors or oscillating circuits into a signal that can be measured by a microcomputer.
Substantial disadvantages of measuring devices of this kind lie in the influencing of the measurement by side-effects, such as for example liquid on the seat part, seat pads, etc. Particularly when the vehicle passenger comes into contact with grounded surfaces or, for example, has on their lap a notebook connected to the vehicle's electrical system, the capacitance alters dramatically, leading to substantial measuring inaccuracies and errors.
Moreover, due to the sensitivity of the sensory system and the large measuring surface in the case of measuring devices of this kind, there is a high degree of sensitivity with respect to electromagnetic disturbances.